Classic switches in telephony networks both provide call control and switching functions. With the emergence of non-circuit associated signalling, it was possible to separate signalling traffic for call control and bearer traffic. The signalling can be transported in a separate network using e.g. signalling system #7 (SS7) protocols. A recent network architecture, also known as the Mobile Softswitch Solution (MSS), introduces softswitches which allow a separation of call control and switching functions into different nodes of the telephony network. This architecture is described in an Internet published whitepaper titled “Efficient Softswitching”, Ericsson AB, August 2006, which can be found at: http://www.ericsson.com/technology/whitepapers/8107_efficient_softswitching_a.pdf. Such a layered architecture enables a separation of service execution, control and connectivity layers, which may be integrated across multiple access networks.
An example of a separation of control and connectivity is a media gateway located between an internet protocol (IP) based network and a public switched telephone network (PSTN) network and being controlled by a media gateway controller (MGC), or a mobile media gateway (M-MGW) being controlled by a mobile switching center server (MSC-S). The M-MGW which is responsible for transporting bearer traffic between e.g. a radio access network (RAN) and a core network (CN), is controlled by the MSC-S using a protocol such as the media gateway control protocol (MGCP), or the MEGACO protocol.
Recent mobile switching center servers may implement a blade cluster architecture, in which each of multiple mobile switching center server blades (MSC-S blades) acts like a fully functional MSC-S. A signalling proxy (SPX) is provided, as a network gateway for connecting remote network nodes in the e.g. radio access network or the core network. As the signalling proxy constitutes a single entry point for e.g. signalling system no. 7 (SS7) based signalling, the internal structure with multiple MSC-S blades is not visible to connecting network nodes. Signalling traffic between the signalling proxy and nodes in the radio access network or the core network is generally relayed by a mobile media gateway wherein signalling messages can be transported between the M-MGW and the MSC-S using a stream control transmission protocol (SCTP) association established between the two entities. Such an architecture is shown in FIG. 7, where the MSC-S 701 comprises plural MSC-S blades 702. MSC-S applications run on these generic processor blades handling signalling traffic. The MSC-S 701 further comprises signalling proxy 703, which hides the blade structure from outside networks. Signalling traffic from RAN 704 and CN 705 are relayed via M-MGW 706. The input/output (I/O) unit 707 is the operations and maintenance (O&M) interface for the MSC-S blades and the signalling proxy. Interfaces 707 may be accessed from the operational support system (OSS) 709.
A common signalling point code (SPC) can be used to address the mobile switching center server 701. Messages addressed to this SPC will be routed to the signalling proxy 703. The forwarding of a message addressed to the SPC to the correct MSC-S blade occurs according to MSC-S internal mechanisms and may consider load distribution between MSC-S blades and the like. This forwarding is generally not visible to external network nodes.
To achieve redundancy for enhancing the security against failure, a second signalling proxy is provided (e.g. 1+1 redundancy scheme). In normal operation, both signalling proxies may share signalling traffic so that 50% of the signalling traffic is routed via the first signalling proxy and the remaining traffic via the second signalling proxy as primary routes. If one of the signalling proxy fails, i.e. the primary route for half the traffic becomes unavailable, the traffic is fully routed via the remaining signalling proxy (alternative route).
In order to be capable of re-routing the signalling traffic via the redundant alternative path, network nodes connecting to the MSC-S must be aware of the two redundant signalling proxies. For many legacy nodes in today's networks, this is not the case. In particular, there are network nodes, such as base station controllers (BSC) or nodes of the public switched telephone network (PSTN), which can only connect to one signalling proxy. Accordingly, there is no possibility for redundant connections for such network nodes. If the signalling proxy to which these nodes are connected fails, then the service to these nodes is discontinued. Calls corresponding to signalling traffic relayed via the failed proxy may accordingly be lost. This may lead to losses in revenue for the operator of the telephony network. Further, the requirements for the reliability of the signalling proxies needs to be increased in order to prevent such failures and resulting losses, which requires more complex signalling proxy architectures and results in higher development costs. Similar problems are encountered for other network nodes, such as a home location register (HLR) or the like.
Accordingly, there is a need to increase the node availability of network nodes, such as switching center servers. In particular, it is desirable to provide signalling proxy redundancy for remote network nodes not capable of connecting to two signalling proxies. It is desirable to provide a signalling proxy redundancy without giving rise to conflicting signalling on the interface between the signalling proxies and the remote network nodes, e.g. network nodes that are not able to address signalling proxies independently.